Thank you for contacting us.As far as I’m aware, there’s no issue with the RTC on the Edison’s latest image. I checked the release notes and there isn’t any mention of any RTC issues either. Would you please share more information on how you’re testing the RTC on the latest image? If you include pictures of your configuration, the commands you’re using and how you’re testing the RTC it’d be of great help.

We have tested the RTC on Edison with image versions 25.5 and release 3.5. and cannot reproduce the described behavior, both versions keep the set RTC. The only difference to your setup is the coin battery that is being used, we used a CR2032 rated at 3.0V with a 210mAh capacity.

Also, please note that the hardware guide states "This backup subsystem allows for interruption of the main supply for a short period of time, such as changing a main system battery", which is what you may be experiencing after ~20 minutes.